1. Field of the Invention
The present invention relates to a solid-state imaging device and a camera apparatus using the same. More specifically, the invention relates to a miniature camera including an image sensor of charge coupled device (CCD) type or complementary metal oxide semiconductor (CMOS) type and commonly built in a mobile telephone or the like.
2. Description of the Related Art
Conventionally, CCD type and CMOS type image sensors (solid-state imaging devices) have widely been employed as miniature cameras built in, for example, mobile telephones. Such an image sensor is known including a plurality of photodiodes, each photodiode has a light converging micro-lens provided on the light incident side thereof for increasing the efficiency of photo detection in an image area.
An incidence angle of light incident on each photodiode after emitted from a camera lens (imaging optical system) is different between the center and the edge of the imaging area. For this reason, the edge of the imaging area (particularly at each of the four corners in case of an area sensor) is lower in the photosensitivity (the efficiency of photo detection) than the center of the imaging area when the micro-lenses and the photodiodes are arranged at equal pitches, i.e., when the photodiodes are aligned with the corresponding micro-lenses in position. This is because there is employed, as the camera lens, a lens having a lens characteristic according to so-called paraxial ray approximation substantially expressed by z/p=tan θ=θ, where θ is an exit angle of chief-ray from a final plane of an optical system; z is an image height; and e is an exit pupil position, while the exit pupil position hardly depends on the image height z. Meanwhile, the p represents an array pitch of the photodiodes.
Upon usage of the camera lens having a lens characteristic according to paraxial ray approximation, the micro-lenses are arranged such that array pitches of the micro-lenses are equally smaller than those of the photodiodes, so that the greater the image height (departing outwardly toward the edge from the center of the imaging area), the greater the dislocation between the photodiode and the micro-lens increases. Namely, the light is made to be obliquely incident on the photodiodes at the edge of the imaging area. Thus, the position of each micro-lens with respect to the position of the corresponding photo diode is gradually offset in the direction of the center as it shifts from the center to the edge of the imaging area. This allows the efficiency of light concentration to be improved over the photodiodes at the edge. Accordingly, the shading at the edge of the imaging area can be corrected, thus ensuring substantially an uniform level of the photosensitivity throughout the imaging area.
As camera lenses have recently been thinned and minimized in overall dimensions, many camera lenses having a lens characteristic deviated from the paraxial ray approximation (such as plastic lenses) have been used. Suppose a case of using, for example, a camera lens having a lens characteristic largely deviated from the paraxial ray approximation such that an exit angle of chief-ray increases as the image height becomes greater and then, the exit angle of chief-ray decreases. In this case, the photosensitivity at the edge of the imaging area is lowered in the case where the micro-lenses are arranged so as to be largely displaced from the photodiodes as shifting from the center of the imaging area.
With the use of an imaging optical system where the exit angle of chief-ray from the final plane of the optical system is not continuously increased as the image height from the optical axis becomes greater, the array pitch of the micro-lenses in the solid-state imaging device is decreased at least at a region between the center and a predetermined position of the edge of the imaging area and is increased at least at a region of the edge exceeding the predetermined position than the array pitch at the predetermined position. Accordingly, the correction of shading at the edge of the imaging area can be suppressed. Such an attempt has been disclosed in the prior art (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-228645).
However, the foregoing attempt permits a plurality of micro-lenses to be arranged at different pitches which are decreased at least at a region between the center and a predetermined position of the edge of the imaging area. When another imaging optical system is used where the exit angle of chief-ray from the final plane of the optical system is increased until the image height rises up to a specific level from the optical axis and then decreased after the specific level of the image height, the correction of shading can hardly be attenuated.
In brief, even when the imaging optical system is used where the exit angle of chief-ray from the final plane of the optical system is increased until the image height rises up to a specific level from the optical axis and then decreased after the specific level of the image height, the paraxial ray approximation is substantially established at a region where the image height is close to zero. Accordingly, the efficiency of light concentration can be ensured when the array pitches of the micro-lenses are substantially equally set smaller than the array pitches of the photodiodes. In contrast, when the micro-lenses are arranged at decremental pitches at least at a region between the zero point and the intermediate point of the image height, the photosensitivity is declined. As a result, the efficiency of light concentration can hardly be maintained throughout the imaging area.